Hello Jerzy,

Thank you for posting in our forum.

You would avoid having to implement a global custom sorting function if you use a custom compare function instead, just for the column that contains the empty strings. This should work when you have multisorting.

There is a StackOverflow thread which describes the same scenario that you have – if you get the snippet from there and use it in your application, the code would look like this:

Define the custom compare function first, so the grid would be able to use it later:  

let customCompare = (a, b) => {
    if(a === "" || a === null) return 1;
    if(b === "" || b === null) return -1;
    if(a === b) return 0;
    return a < b ? -1 : 1;
}

Then, in your grid configuration: 

$("#grid").igGrid({
    dataSource: data,
    primaryKey: "ProductID",
    autoCommit: true,
    columns: [   
            { headerText: "Product Category", key: "ProductCategory", dataType: "string", width: "25%" },
   ///     ...some column definitions...
    ],
    features: [
        {
            name: 'Sorting',
            mode: "multi",
            columnSettings: [
                {
                    columnKey: "ProductCategory",
                    compareFunc: customCompare
                }
            ]
        }
    ]
})

I have attached an isolated code sample that demonstrates this approach, which you may run on your side if you want to:

igGridCustomComparer.zip

Try sorting the Product Category column and the empty records are being placed at the very bottom of the grid. 

Please let me know if you need any further assistance.